A spatially resolved measurement of T2 relaxation (T2 mapping) is one of the most basic magnetic resonance imaging (“MRI”) core analysis measurements employed to determine a wide variety of fluid/matrix properties. MRI employing T2 distribution measurements, including T2 distribution mapping, is an appealing technique for chemical and petroleum engineering, including core analysis, due to its ability to probe the occupancy of pores by water and oil phases [1]. It is suitable, in principle, for studies of a variety of miscible and immiscible processes, including enhanced oil recovery, and for characterizing porous rocks with regard to mass transfer between flowing and stagnant fluids. Recently, it has been recognized to be a promising technique for spatially resolved analysis of the irreducible water saturation of porous rocks [2]. T2 distribution measurements, including T2 distribution mapping, are also widely adopted in clinical applications as well.
It is desirable that a T2 mapping scheme provide as wide an interval of measurable T2 as possible for a comprehensive analysis of relaxation data. Ideally, spatially resolved T2 measurements are expected to give as realistic T2 distributions as regular bulk CPMG measurements. High sensitivity signal-to-noise ratio (“SNR”) measurements are also important since low field magnets are traditionally used for core rock analysis.
Two pulse sequences for one-dimensional (“1-D”) T2 mapping which employ phase encode magnetic resonance imaging techniques, namely CPMG-prepared SPRITE and spin-echo single-point imaging (“SE-SPI”) are described in [3]. The CPMG-prepared SPRITE sequence has no hardware restrictions on the echo timing other than those for a regular CPMG experiment but is relatively slow, as the measurement time is proportional to T2 dimension, and has a worse SNR due to a small radio frequency (“r.f.”) pulse flip angle. The spin-echo SPI provides much faster measurements and with good SNR, but has a restriction on the first echo acquisition time. The latter makes it difficult to measure short T2 (<1 ms) characteristic, e.g., for water in clay-containing rocks and cement-based materials.
Prior art MRI based methods are unable to reliably measure short lifetime signal components in a T2 distribution.